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Doctoral thesis, 2018

Endocrine response to illness in sick horses

Stewart, Allison

Abstract

Endocrinological responses are under tight physiological control in healthy animals, with homeostatic adaptions made for diet, exercise and illness. Transient adrenal dysfunction with inadequate cortisol production, known as relative adrenal insufficiency (RAI), is a major component of critical illness–related corticosteroid insufficiency (CIRCI), which involves dysfunction of the entire HPA axis. Recognition and treatment of CIRCI improves human survival rates. Whereas a low-dose ACTH stimulation test appears to be the most sensitive method of diagnosing CIRCI in people, published doses for this test in horses were considerably higher, and the lowest dose of ACTH that will maximally stimulate the adrenal glands of horses was unknown. Critical illness is also associated with peripheral tissue insulin resistance and hyperglycaemia in many species, but insulin and glucose dynamics in spontaneously sick adult horses remains to be investigated. The overall aim was to document changes in cortisol, ACTH, insulin and glucose in sick horses; determine if adrenal or pancreatic dysfunction occurred, and which parameters were correlated with non-survival. A secondary aim was to validate the low-dose ACTH stimulation test in healthy adult horses and neonatal foals and then determine if CIRCI could be identified in sick horses. A dose of synthetic ACTH of 0.1 µg/kg in healthy adult horses and 0.25 µg/kg in neonatal foals resulted in maximal adrenal stimulation. In sick horses, cortisol, ACTH and ACTH/cortisol ratios were higher in non-survivors at admission. ACTH/cortisol ratios were higher in horses with systemic inflammatory response syndrome (SIRS) and those with an ischemic gastrointestinal lesion, suggesting adrenal dysfunction. Although horses with ischemic lesions had higher basal cortisol, they failed to respond to ACTH stimulation, further indicating adrenal dysfunction. RAI/CIRCI was diagnosed in 21% of sick horses and is associated with a poorer prognosis. Horses presenting with hyperglycaemia and corresponding relative hypoinsulinemia (suggestive of endocrine pancreas dysfunction) had a worse prognosis.

Keywords

endocrinology, horses, adrenal, equine, inflammation, colic, CIRCI, cortisol, adrenocorticotropic hormone, ACTH stimulation, SIRS, insulin dysregulation, pancreas

Published in

Acta Universitatis Agriculturae Sueciae. Veterinaria
2018, number: 2018:59
ISBN: 978-91-7760-258-3, eISBN: 978-91-7760-259-0
Publisher: Department of Clinical Sciences, Swedish University of Agricultural Sciences

Authors' information

Stewart, Allison
Swedish University of Agricultural Sciences, Department of Clinical Sciences

UKÄ Subject classification

Clinical Science

URI (permanent link to this page)

https://res.slu.se/id/publ/104221